As electronic devices continue to be miniaturized, the mounting of a microelectronic substrate to another microelectronic substrate may become increasingly difficult. Thus, high density, high performance and reliable interconnection techniques are desirable for microelectronic substrates. It will be understood by those having skill in the art that microelectronic substrates include but are not limited to integrated circuit chips, wafers, printed circuit boards, multi-chip module substrates, ceramic substrates, glass substrates and any other electronic packaging substrates that are used in microelectronic systems including optoelectronic systems.
In order to satisfy these requirements, solder bump technology is increasingly being used for microelectronic packaging. Solder bump technology is also referred to as C4 (controlled collapse circuit connection) or "flip chip" technology. Solder bump technology can allow a bare integrated circuit to be mounted on a next level package, without requiring encapsulation of the integrated circuit. Moreover, solder bump technology can utilize the entire area of a microelectronic substrate for connection rather than only using the periphery thereof. High performance may be obtained because high frequency signal transmission may be obtained due to direct bonding.
Conventional solder bumps generally use an alloy of lead (Pb) and another metal. Specifically, an alloy of 95% lead and 5% tin (Sn) is widely used. Unfortunately, the use of the lead may be harmful for the environment, so that alternatives to lead are being investigated. Alloys of silver (Ag) and tin (Sn) have been investigated as alternatives to lead-based alloys for solder bumps. However, since solder bumps are generally plated on an underbump metallurgy using a mask, it may be difficult to plate silver-tin alloy solder bumps as opposed to lead-tin solder bumps. Although it may be possible to use paste screening processes to form silver-tin alloy solder bumps, plating may be preferred for high-density solder bump interconnections.